The present invention relates to the fabrication of structural shapes of composite materials, particularly to the fabrication of aircraft engine blades made of boron fiber and aluminum.
Composite materials, such as the combination of boron fibers in an aluminum metal matrix, are of interest to aircraft designers because of their high specific strength and specific stiffness. However, because of these properties and the brittleness often associated with the fiber material, they are very difficult to fabricate. The composite material is generally provided as a raw material in the form of a flat sheet, having one layer of fibers. The typical component is comprised of a multiplicity of layers or sheets which have been compacted together to form a unitary object bound by the matrix material. The consolidation is generally accomplished by placing a multiplicity of precut sheets of varying configuration and orientation within a rigid die and applying sufficient force and temperature to densify the article.
The foregoing procedure works well when the object has a regular cross section, but there is considerably more difficulty when the object has a varying cross section and contour, such as is characteristic of a compressor blade of a gas turbine engine as described further herein. When the flat sheets of composite material are assembled and placed in the contoured die they (a) do not fit properly, and (b) tend to shift out of position as the dies are closed and the contouring forces are applied. Since the typical fiber does not, by virtue of its choice, have appreciable ductility or deformability compared to the matrix material, the mis-positioning of fibrous material will result in a defective part, with there being excess material in one location and insufficient material and densification in another. Various techniques such as tack welding and fixturing have been employed to try to overcome these problems, but on the whole they have not been satisfactory. A further difficulty in the working of many composite materials which are consolidated by hot pressing is that the pressing temperature and time must be limited in order to avoid deleterious interaction between the fibers and the matrix material.
Additionally, on most composite material structures which are subjected to severe environments a protective cladding or skin is applied. Such is the case with gas turbine compressor airfoils, wherein a sheet of metal such as titanium is generally clad to the surface of boron aluminum composite airfoils. This sheet is best applied during the consolidation process of the core segment and this adds a further variable which must be coped with.
The present invention will be seen to relate to the use of gas pressure for preforming composite material cores and skins prior to their being bonded as a finished assembly. Gas pressure has been previously used to form sheet metal objects, as is disclosed in various U.S. patents. Wisberger U.S. Pat. No. 3,024,525 and Somers et al. U.S. Pat. No. 3,895,436 describe the internal pressurization of double wall objects to expand the space between the walls and thus free-form useful hollow structures. Larson U.S. Pat. No. 4,077,109 describes a similar procedure, but with the expansion taking place within a die, thereby producing an object with a definite contour determined by the die. Hamilton et al. in U.S. Pat. Nos. 3,934,441, 3,920,175, and 3,927,817 describe the combined shaping and bonding of hollow metal objects by the use of gas pressurization in a hard die. Schier et al. in U.S. Pat. No. 4,087,037 show the use of a flexible die or pressure membrane to form a contoured structure against a hard die. In U.S. Pat. No. 3,701,190 Stone, Jr., an inventor herein, discloses a method of compacting a composite airfoil using a pressurized membrane. Except for the last mentioned patent, the prior art does not relate to the forming of composite containing structures nor is it suggestive of ways of overcoming the problems described above. The last mentioned patent disclosure comprises a method wherein the airfoil is compacted and shaped by the conventional rigid die, but with the addition of a membrane laid against one of the die halves. Gas pressure applied to this membrane aids in the final compaction of the structure. But prior to the instant invention there was no appreciation of any means by which either the skin or composite core material could be advantageously and effectively preformed prior to its being placed in the final compaction die.